Regulation and function of transcription factor MEF2C in vascular development

Blood vessel development in vertebrates starts early during embryogenesis with the de novo formation of blood vessels through the process of vasculogenesis. During vasculogenesis, endothelial progenitor cells coalesce into tubular structures. Pericytes and smooth muscle cells then surround these tubes to form functioning blood vessels. Myocyte enhancer factor 2 (MEF2) transcription factors have been shown to play a critical role in the differentiation of cardiac and skeletal muscle. MEF2C is also expressed in several other cell types, including smooth muscle and endothelial cells, yet little is known about the transcriptional pathways in which MEF2C participates, particularly during vascular development.; Before the research contained within this dissertation, no transcriptional targets of MEF2 proteins had been discovered in smooth muscle. However, we found a completely conserved MEF2 site upstream of the HRC transcriptional start site that proved to be necessary for expression of a lacZ transgene in skeletal, cardiac, and arterial smooth muscle; identifying HRC as the first known target gene of MEF2 in smooth muscle. We also found that this HRC enhancer was unresponsive to serum response factor (SRF), making HRC one of only a few smooth muscle genes known not to be a transcriptional target of SRF.; To understand the transcriptional pathways that MEF2C participates in, we initiated an enhancer screen and used evolutionary conservation to identify multiple tissue-specific modular enhancers for mef2c. In this screen, we identified a skeletal muscle enhancer, an anterior heart field enhancer, three neural crest enhancers, and two endothelial enhancers at the mef2c locus. One of the enhancer fragments we discovered was a 5.6 Kb fragment that is sufficient to direct expression to both the neural crest and endothelium. We have subsequently separated the neural crest and endothelial elements into two independent enhancers and pared down the endothelial element to a mere 44 base pairs, which are both necessary and sufficient to direct expression of the lacZ reporter to endothelial cells. Within this 44 bp we have found two ETS factor binding sites and an E-Box that are necessary for endothelial expression, and we continue to search for other regulatory elements.